Power-shiftable gear, especially two-speed planet gear

ABSTRACT

The invention relates to a power-shiftable two-speed gear. The gear is designed as planet gear (3). The two ratios can be hydraulically shifted via two friction clutches (9 and 10). At least one of the friction clutches (9 or 10) is fitted with a mechanism which prevents the full locking pressure from acting upon the friction components during downshifts or upshifts. A store (30), which the torque curve can be affected, is used for this purpose. To prevent traction loss and abrupt changes which are found to be particularly unpleasant, the store (30) consists of a cylinder (32) and a piston (33) is fitted in the line (18 and 24) leading to the pressure chamber (17 and 23). A non-return valve (28) with a choke (29) in parallel with it is integrated into the piston.

The invention concerns a power-shiftable gear, especially a two-speedgear preferably designed as planet gear. The ratios of said gear areshifted by hydraulically actuatable friction clutches. The frictioncomponents (inner and outer discs) of the friction clutches are engagedby spring tension and disengaged by oil pressure.

In a power-shiftable gear disclosed in WO 89/08796, two speeds can beshifted. When shifting the speeds, friction clutches (one of saidfriction clutches acts as brake) are alternatively disengaged andengaged. The friction clutches are engaged by plate springs. When bothfriction clutches are pressureless, that is, engaged, the input andoutput (parking brake) are blocked.

Due to the relatively strong forces of the plate spring, the pressuremedium (hydraulic oil) is removed from the respective pressure chambersto the tank within a very short time. The time needed for disengagingthe friction clutches depends on the delivery rate of a source ofpressure medium (pump).

By inserting non-return valves with chokes in parallel therewith in theline leading to the pressure chambers, it can be ensured that thealternative disengaging and engaging of the friction clutches beadjusted in a manner such that the friction clutch to be engaged isengaged only when the friction clutch to be disengaged is disengaged.

The non-return valves, added around the chokes, are disposed so thatupon opening the oil current can flow unchecked into the respectivepressure chamber. When closing, the oil current removed from thepressure chamber is choked drawn into the tank.

The moment of closing can be affected by the valves mentioned. Theengaging operation itself, that is, the timed torque intake of thefriction clutch, cannot be affected thereby. As soon as the pistonworking upon the friction components in the sense of engaging hasovercome the clearance (sum of the spacings of the inner and outer discsof a friction clutch in a disengaged state), the torque transmissible bythe friction components suddenly increases to a maximum valve. Since theengaging operation takes place in a very short time, this makes anabrupt change itself noticeable, especially during a downshift from thesecond to the first speed, which is felt as especially unpleasant.

An older not previously published proposal of the applicant(international patent application PCT/EP93/02421)-which is the point ofdeparture of the instant invention-relates to means which prevent,during a gear change, that the full locking force acts first upon thefriction components of the friction clutches. Thus, the torque curvecan, in particular, be affected so as to avoid an abrupt change.

This invention is based on the problem of further improving the pressureand torque curves when downshifting or upshifting.

The stated problem is solved by a power-shiftable gear, especially atwo-speed gear designed as planet gear, having at least one frictionclutch the friction components of which are engaged by spring tensionand disengaged by oil pressure, one line leading to a pressure chamberand in which is a store formed by one cylinder and one spring-loadedpiston with a non-return valve and a choke in parallel with it.

Hereby is prevented, in the first place, that when clearance between thefriction components is reduced to a value of zero or near zero, the fullspring tension act upon the friction components. In addition, the storeis situated in the line leading to the pressure chamber of the frictionclutch in a manner such that during engaging the friction clutch thephase lasting until the torque begins to be taken over is shortened.Loss of traction practically does not occur.

In a preferred embodiment, when the friction clutch is disengaged, thepiston assumes a position in which the store has a minimum of storagecapacity.

Since the store is practically empty when the friction clutch isdisengaged, it is very advantageously possible during the beginning ofengaging of the friction clutch, that is, with the line connected withthe tank and engaged non-return valve, immediately to convey a part ofthe pressure medium that is in the pressure chamber into the store. Thestore then has a maximum storage capacity.

A steady torque curve results when the residual pressure mediumremaining, in the pressure chamber flows off, via the choke, during thefurther course of the engaging movement of the friction clutch.

It is especially advantageous if the ratio of capacity of the pressurechamber when the friction clutch is disengaged to the maximum storecapacity is chosen to be substantially 2:1. By selecting an adequateratio of both capacities it is easily possible to vary the time thatelapses until beginning of assumption of the torque.

Other features essential to the invention and the advantages resultingtherefrom are to be understood from the description that follows of anembodiment. In the drawings:

FIG. 1 diagrammatically shows a cross section through a power-shiftablegear and the elements required for shifting the gear;

FIG. 2 is the power-shiftable gear of FIG. 1 in a different shiftposition;

FIG. 3 is a diagram reproducing the pressure curve over the course oftime for a downshift/upshift; and

FIG. 4 is a diagram showing the torque curve for a downshift/upshift.

The gear diagrammatically shown in cross section in FIG. 1 is ahydraulically power-shiftable two-speed gear. An input shaft 2 isrotatably situated in a housing 1. The input shaft 2 is driven by aprime mover, itself not shown. This can be, for example, a continuouslyadjustable hydraulic motor or an electromotor. The hydraulicallypower-shiftable gear is preferably designed as planet gear 3. The planetgear 3 is specifically comprised of one inner central gear 4, one outercentral gear 5, several planet gears 6 and a planet carrier 7. Theplanet gears 6 are rotatably supported on the planet carrier 7, whilethe latter is non-rotatably connected with an output shaft 8. The outputshaft 8 can be in driving connection with a spur gear stage, itself notshown, by which an output shaft is actuated for moving the gears of aloader or the like.

Both ratios of the power-shiftable gear are hydraulically shiftable viatwo friction clutches 9 and 10. The outer friction clutch 10 is herepreferably situated concentrically in relation to the inner frictionclutch 9 and designed as brake. Regardless of its function as a brake,this part is designated as a friction clutch.

The outer central gear 5 has two disc carriers 11 and 12. Said disccarriers 11 and 12, together with the outer central gear 5, arepreferably made as a one-piece component.

This disc carrier 11 lying internally in the drawing, non-rotatably andaxially movably accommodates, in a manner known per se, outer discs 13of the friction clutch 9. Inner discs 14 belonging thereto are situatedon the input shaft 2 non-rotatably and axially movably, as known per se.

A pot-shaped piston 15 is axially movably passed into the housing 1.Said piston is actuated by several plate springs 16 so as to bedisplaced in a closing direction toward the outer and inner discs 13 and14 of the friction clutch 9. Within the interior of the piston 15 is apressure chamber 17 to which leads a first line 18 through whichpressure medium, such as oil, is supplied and removed.

From what has already been said, it follows that when the pressurechamber 17 is pressureless, the friction clutch 9 is engaged by thetension of the plate springs 16 which loads the piston 15 and moves itto the right-based on the drawing. If, on the contrary, the pressurechamber 17 is pressurized with a pressure medium, the friction clutch 9is disengaged against the tension of the plate springs 16, that is, thepiston 15 moves to the left-based on the drawing.

The friction components of the friction clutch 10 consist of inner discs19 which are retained upon the disc carrier 12 of the outer central gear5 and outer discs 20 situated in the housing 1, in a manner known perse. For disengaging and engaging the friction clutch 10, another piston21 is provided which is sliding passed axially into the housing 1 andupon which acts a tension exerted by plate springs 22. A line 24 leadsfrom a store 30 to a pressure chamber 23.

As can be seen from the drawings, the friction clutch 9 is in an engagedstate while the friction clutch 10 is disengaged. The second speed isengaged. The torque of the input shaft, on one hand, is passed via theengaged friction clutch 9 and the outer central gear 5 to the planetgears 6 and, on the other hand, via the inner central gear 4 to theplanet gears 6. The planet carrier 7 then transmits the added up powerto the output shaft 8.

The lines 18 and 24 are connectable either with a pump 26 or with a tank27 via a gear-change valve 25 designed as 4/2 directional valve. Asshown in the drawing, between the gear-change valve 25 and the pressurechamber 17, in the line 18, can be inserted a non-return valve with achoke in parallel therewith. The non-return valve allows an unhinderedpressurization of the pressure chamber 17 with pressure oil, while thereturn of the oil current from the pressure chamber 17 to the tank 27 ischoked.

The line 24 differs from the above explained design. A first part ofsaid line is attached to the store 30.

The store 30 basically consists of one cylinder 32 into which a piston33 is axially movably passed.

The piston 33 is spring loaded, for example, by a compression spring 34.The piston 33 divides the store 30 in two compartments. The first partof the line 24 discharges in the first compartment appearing to theright in the drawing (in which is housed the compression spring). Asecond part of the line 24 leads from the second compartment, appearingto the right, in the drawing to the pressure chamber 23 of the frictionclutch 10.

The first and the second parts of the line 24 are interconnected by aduct 31. The duct 31 can be formed, for example, by holes whichpenetrate the piston 33. The duct 31 can be understood as a componentpart of the line 24.

In the piston, specifically within the duct 31, is integrated anon-return valve 28 with a choke 29 in parallel with it. When thefriction clutch is disengaged (see FIG. 1), the pressure medium flowsfrom the pump 26 via the gear-change valve 25 to the store 30 and whenthe non-return valve 28 is open, it flows through the duct 31 to thepressure chamber 23. The piston 33 assumes its left end position shownin the drawing. In this position, practically no pressure mediumaccumulates in the store 30. The friction clutch 10 is disengaged: thesecond speed is engaged.

When downshifting from the second to the first speed, the gear-changevalve 25 changes its shift position and assumes the shift position shownin FIG. 2. The line 18 is connected with the pump 26. When downshifting,the pressure chamber 17 is filled with pressure medium. The piston 15moves to the left against the tension of the plate springs 16. Thefriction clutch 9 is disengaged.

With the change of the gear-shift position of the gear-change valve 25,a partial volume of the pressure medium engaged in the pressure chamber23 of the friction clutch 10 is transferred immediately to the cylinderspace-lying to the left of the store 30. The store capacity 35 reaches amaximum practically without delay. The volume of the pressure mediumstill remaining in the pressure chamber amounts substantially to onehalf of the output volume or, expressed in other words, the storeimmediately takes over one half of the output volume. The frictioncomponents (inner and outer discs 19 and 20) of the friction clutch 10move here toward each other until the release clearance finally assumesa zero value. This shifting state is shown in FIG. 2.

In order to prevent the friction clutch 10 from suddenly transmittingthe full torque when the release clearance of the friction components isreduced to the value of zero, the pressure medium enclosed in the store30 (store capacity 35) now flows out (the non-return valve 28 isengaged) via the choke 29 and the line 24 to the tank 27. The time curvecan be affected by the characteristic line of the compression spring 34and the design of the choke 29. The store 30 takes over a retainingfunction in a manner such that a retaining pressure remains in the line24 between the pressure chamber 23 and the non-return valve 28. Saidretaining pressure is reduced not suddenly but steadily by the removalof the pressure oil enclosed in the store 30 via the choke 29. Due tothe steady reduction of said retaining pressure, the friction clutch 10becomes finally completely engaged with a steady transition. In saidtransitional phase (slip phase of the friction components of thefriction clutch 10), the piston 33 moves to the left under the tensionof the compression spring 34 until the store capacity 35 has reached aminimum value. The plate springs 22 convert the piston 21 to its finalengaged position.

The pressure curves over the course of time are to be understood fromthe diagram of FIG. 3. At the moment t1, the downshift is started. Thegear-change valve 25 is reversed. The drawn lines apply to the pressurecurve in the pressure chamber 23. From the moment t1 to the moment t2,the store capacity 35 is built up. The phase of a traction loss (periodof time t1 to t2) is accordingly extremely short. During the period oftime from t2 to t3, the friction clutch 10 is engaged by a steadyreduction of pressure so that the gear-shift operation proceedssmoothly. Starting from the moment t3, the pressure in the pressurechamber 23 drops quickly to zero.

The torque curve of the downshift is to be understood from the diagramin FIG. 4. The engaging friction clutch 10 starts absorbing the torqueat the moment t2. Without the store, the torque increase-at a latermoment, that is, with a longer phase of a traction loss-would proceedvery abruptly, as shown by the dotted line.

The engaging process can be affected in several respects. A timed changeis possible by varying cross section of the choke 29. The engagingprocess can be affected by the design of the characteristic line of thecompression spring 34 (the use of a different adequate spring isobviously possible) and by the store capacity 35.

The invention can also be applied to rotary friction clutches.

Finally let it be pointed out that the arrangement according to theinvention, can equally be used both in downshift and upshift processes.

    ______________________________________                                        Reference numerals                                                            ______________________________________                                        1      housing       19      inner discs                                      2      input shaft   20      outer discs                                      3      planetary gear                                                                              21      piston                                           4      inner central gear                                                                          22      plate spring                                     5      outer central gear                                                                          23      pressure chamber                                 6      planetary gear                                                                              24      line                                             7      planetary gear carrier                                                                      25      gear-change valve                                8      output shaft  26      pump                                             9      friction clutch                                                                             27      tank                                             10     friction clutch                                                                             28      non-return valve                                 11     disc carrier  29      choke                                            12     disc carrier  30      store                                            13     outer discs   31      duct                                             14     inner discs   32      cylinder                                         15     piston        33      piston                                           16     plate spring  34      compression spring                               17     pressure chamber                                                                            35      store capacity                                   18     line                                                                   ______________________________________                                    

I claim:
 1. A power-shiftable gear, especially two-speed gear designedas planet gear (3), having at least one friction clutch (9 and 10) thefriction components of which (13, 14 and 19, 20) are engaged by springtension (16, 22) and disengaged by oil pressure, having a line (18 and24) which leads to a pressure chamber (17 and 23) and in which is astore (30) comprising of one cylinder (32) and one spring-loaded piston(33) with a non-return valve (28) and a choke (29) in parallel with it.2. A gear according to claim 1, characterized in that said piston (33),when said friction clutch (9 and 10) is engaged, assumes a position inwhich the store has a minimum storage capacity (35).
 3. A gear accordingto claim 1, characterized in that in said store (30), at the beginningof engaging said friction clutch (9 and 10) when said line (18 and 24)is connected with said tank (17) and said non-return valve (28) isengaged, part of the pressure medium is transmitted into said pressurechamber (17 and 23), said store (30) has a maximum storage capacity(35).
 4. A gear according to claim 3, characterized in that the residualpressure medium remaining in said pressure chamber (17 and 23)discharges via said choke (29) during the further course of the closingmovement of said friction clutch (9 and 10).
 5. A gear according toclaim 3, characterized in that a ratio of the capacity of said pressurechamber (17 and 23), when said friction clutch (9 and 10) is disengaged,to the maximum store capacity (35) is about 2:1.
 6. A gear according toclaim 4, characterized in that a ratio of the capacity of said pressurechamber (17 and 23), when said friction clutch (9 and 10) is disengaged,to the maximum store capacity (35) is about 2:1.